The overall goal of this longitudinal study will be to examine psycho-physiological mechanisms that explain, (a) level of physical activity participation and (b) spontaneous change in physical activity over a two- year period in a cohort of 848 young adults. An exercise response index (ERI) will be calculated that represents the relationship between an individual's perceptual response to exercise and the physiological response to exercise while undergoing a progressive cycle ergome_er test. The perceptual responses include rating of perceived exertion (RPE), global mood and exertional pain. Physiological responses include oxygen oxygen consumption, carbon dioxide production, heart rate, pulmonary ventilation, and lactic acid concentration. We anticipate that low levels of physical activity or inability to maintain physical activity level is associated with a maladaptive uncoupling of the expected relation between exertional perceptions and the underlying physiological mediators, thus, making physical activity undesirable. The subjects for this study will include 848 individuals aged 27-30 years who are currently participating in a longitudinal study. Data will be collected at two time points approximately two years apart. Data will include metabolic, cardiovascular, and perceptual responses to an exercise stimulus at submaximal workloads. Subjects wiU also complete an assessment of their current physical activity with a questionnaire. Based on the two assessments of physical activity, subjects will be classified as "adopter", "maintainer", "drop-out" or "non-adopter" at the second assessment based on their activity level at the first assessment. A trained interviewer will conduct a one-on-one semi-structured interview with each participant to identify factors that contributed to any change in the pattern or amount of physical activity. Additional questionnaires will be completed to assess behavioral, environmental, psychological, and social factors that have been shown to be related to physical activity.